A semiconductor chip is usually accommodated (sealed) in a container called a package to be shielded and protected from outside air, and mounted on a substrate as a semiconductor element. For the package, a curable resin such as a thermosetting resin such as epoxy resin is used. As a method for producing a semiconductor element, for example, a so-called transfer molding or compression molding method is known wherein a semiconductor chip or the like is disposed so as to be positioned at a predetermined location in a mold, and a curable resin is filled into the mold and cured.
In recent years, in a semiconductor element, for the purpose of improving heat dissipation or thinning, for example, it has been in practice to expose a semiconductor chip or a source electrode bonded to a semiconductor chip (e.g. Patent Document 1). As such a semiconductor element, a sensor may typically be mentioned.
In recent years, as a CCD (Charge Coupled Device) image sensor, a CMOS (Complementary Metal Oxide Semiconductor) image sensor or the like, it has been common to employ a semiconductor element having a hollow structure in which a sealing glass is laminated via a rib member (spacer) on a substrate on which a semiconductor chip is mounted. In the production of such a semiconductor element, after laminating a sealing glass on the rib member, it is common to further seal the circumferential surface of the rib member and the sealing glass with a curable resin (e.g. Patent Document 2).
Such a semiconductor element (hereinafter referred to also as an “exposed element”) having a part of a semiconductor chip or another site (such as a source electrode or a sealing glass) exposed, may be produced, for example, by pressing a mold to a portion to be exposed of the semiconductor chip or another site (hereinafter referred to also as an “exposed portion”) and, in that state, filling and curing a curable resin.
However, in the production of an exposed element, leakage of the curable resin to the exposed portion (so-called burr) is often problematic. That is, the filled curable resin enters between the mold and the exposed portion and gets cured in that state, so that the surface of the exposed portion is likely to be thinly covered by the cured product. In order to reduce the leakage, if the clamping pressure of the mold is increased to strongly press the mold to the exposed portion, the exposed portion is likely to be dented or damaged, so that the semiconductor chip or another site will be broken. Therefore, it is necessary to let the exposed portion and the mold be in contact weakly with each other, whereby leakage is likely to occur. If such leakage occurs, it will be required to take a step of removing the cured product covering the exposed portion, whereby the productivity will be lowered.
As a countermeasure against such leakage, it has been proposed to use mold release films of the following (1) to (4).
(1) one comprising two layers i.e. a layer (layer A) providing releasability from a molded article and a layer (layer B) providing heat resistance against heating during molding (Patent Document 3).
(2) one comprising at least two layers i.e. a layer (layer A) providing releasability of a semiconductor element and a layer (layer B) providing heat resistance against heating during molding, at the time of resin molding a semiconductor element, wherein layer B is a polyamide resin (Patent Document 4).
(3) one made of a composite film comprising a substrate film and an auxiliary layer having a compression elastic modulus lower than the substrate film, and to be used so that the auxiliary layer faces the surface of a chip to be sealed (Patent Document 5).
(4) one having formed on a substrate sheet a cushion layer having releasability with respect to a molding resin and having a compression elastic modulus of from 15 to 300 MPa at 180° C. and a thickness of from 20 to 70 μm (Patent Document 6).
In the mold release film (1), for the layer A, an acrylic resin, polyisobutylene, a poly(4-methylpentene-1) resin or a syndiotactic polystyrene resin is used, and for the layer B, a poly(4-methylpentene-1) resin, biaxially stretched polyethylene terephthalate, a soft copolymerized polyethylene terephthalate resin, polyether sulfone or polyimide is used.
In the mold release film (2), for the layer A, an acrylic resin or a silicone resin is used.
In the mold release film (3), for the auxiliary layer, a fluororesin (polytetrafluoroethylene, etc.) or fluorine rubber is used, and for the substrate film, a polyimide film or an aluminum foil is used.
In the mold release film (4), for the cushion layer, silicone rubber is used, and for the substrate sheet, polyester is used.